1. Field of the Invention
This invention relates to a halogen exchange process for the manufacture of hydrochlorofluoropropanes substantially free of the production of perhalogenated and carbon--carbon cleavage by-products. More specifically, but not by way of limitation, the present invention relates to the use of high-fluoride-content antimony pentahalides for the preparation of CClF.sub.2 CF.sub.2 CHCl.sub.2 (HCFC-224ca), CCl.sub.2 FCF.sub.2 CHClF (HCFC-224cb), CF.sub.3 CF.sub.2 CHCl.sub.2 (HCFC-225ca), CClF.sub.2 CF.sub.2 CHClF (HCFC-225cb) and CF.sub.3 CF.sub.2 CHClF (HCFC-226ca) free of chloroform.
2. Description of Related Art Including Information Disclosed Under .sctn..sctn.1,97-1.99
Hydrogen-containing fluorocarbons and chlorofluorocarbons (HFCs and HCFCs, respectively) are of current commercial interest as potential environmentally safe replacements for existing perhalogenated products which are now recognized as having high-ozone depletion potential. For example, the widely used solvent 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113, CCl.sub.2 FCClF.sub.2) is suspected of contributing to the depletion of ozone in the earth's stratospheric layer. Recently, certain dichloropentafluoropropanes, C.sub.3 HCl.sub.2 F.sub.5, such as CF.sub.3 CF.sub.2 CHCl.sub.2 (HCFC-225ca), CF.sub.3 CCl.sub.2 CHF.sub.2 (HCFC-225aa), and CClF.sub.2 CF.sub.2 CHClF (HCFC-225cb) possessing properties similar to those of CFC-113, have been suggested as potential replacements for CFC-113. Because these C.sub.3 HCl.sub.2 F.sub.5 compounds have a hydrogen substituent on a carbon atom adjacent to a halogen-substituted carbon atom, they can undergo dehydrohalogenation in the troposphere with little or none of these compounds surviving to reach the stratosphere.
Dichloropentafluoropropanes, C.sub.3 HCl.sub.2 F.sub.5, notably CF.sub.3 CF.sub.2 CHCl.sub.2 (HCFC-225ca) and CClF.sub.2 CF.sub.2 CHClF (HCFC-225cb), have been prepared by the reaction of dichlorofluoromethane, CHCl.sub.2 F, with tetrafluoroethylene, CF.sub.2 =CF.sub.2, in the presence of aluminum chloride [The Prins reaction: Joyce, U.S. Pat. No. 2,462,402; Coffman et al., J. Am. Chem. Soc. 71 979-980 (1949); Paleta et al., Coll. Czech. Chem. Comm. 35 1867-1875 (1971)]. This process suffers the disadvantage that extensive halogen exchange also takes place between the organic components and the aluminum chloride catalyst. One aspect of such an exchange is that substantial amounts of chloroform, CHCl.sub.3, are produced from CHCl.sub.2 F. Chloroform is not only capable of entering into a competitive Prins reaction with CF.sub.2 =CF.sub.2 (to produce a tetrafluoro derivative) but forms azeotropes with the pentafluoro products which prohibit subsequent purification thereof by simple distillation. The CHCl.sub.3 contaminant is objectionable and must be removed. As a result, not only is the yield of the desired pentafluoro compounds lowered, but further costly processing is required to recover them substantially free of by-product CHCl.sub.3.
In a recent U.S. Pat. No. 4,851,595, a process for preparing a hydrogen-containing fluoroethane, CF.sub.3 CH.sub.2 F, using a liquid-phase SbF.sub.5-x Cl.sub.x, where x=0 to 2, halogen exchange agent in a stoichiometric quantity sufficient to provide at least one fluoride for every chloride replaced in a chlorinated underfluorinated precursor under mild reaction conditions is disclosed. Since this reference deals exclusively with hydrochlorofluoroethanes, the above-mentioned problem associated with the C.sub.3 HCl.sub.2 F.sub.5 azeotropes with CHCl.sub.3 is not dealt with, but the reference does acknowledge that, depending primarily on the antimony pentahalide and reaction temperature employed, various by-products from side reactions involving carbon--carbon cleavage, chlorination and subsequent fluorine-for-chlorine radical exchange will be present.
Antimony pentafluoride (SbF.sub.5) has also been employed as a halogen exchange agent in other processes. For example, Benning et al., U.S. Pat. No. 2,490,764, disclose its use for fluorinating compounds of the formula ACF.sub.2 (CF.sub.2).sub.n CF.sub.2 B, where n is an integer of at least 1, A is H or Cl and B is Cl or F, to produce compounds of the formula ACF.sub.2 (CF.sub.2).sub.n CF.sub.3, where n is at least 1 and A is H, Cl or F. The highly fluorinated starting materials (propanes and higher) are extremely inert and require high temperatures (175.degree.-350.degree. C.) for their conversion to the more highly fluorinated derivatives. It appears, however, that side reactions leading to low-boiling by-products can occur under the rather stringent reaction conditions employed, notably with short-chain reactants. In Example III, designed to prepare a heptafluoropropane, H(CF.sub.2).sub.3 F (b.p. -18.degree. C.), by reaction of a chlorohexafluoropropane, H(CF.sub.2).sub.3 Cl (b.p. 21.degree. C.), with SbF.sub.5 at 175.degree. C. for 10 hours, the recovery of organic material, consisting of two main fractions, was only 70% by weight of the material charged. One of these fractions was H(CF.sub.2).sub.3 F, the other unreacted H(CF.sub.2).sub.3 Cl; relative amounts not disclosed. Unidentified lower-boiling fractions (-40.degree. C. to -18.degree. C.) were also obtained which strongly suggests that carbon--carbon cleavage to low-boiling C.sub.1 and C.sub.2 compounds and/or fluorination to perfluorinated F(CF.sub.2).sub.3 F (b.p. -38.degree. C.) had occurred in addition to the desired halogen exchange reaction.
The prior art recognizes such difficulties attending the use of SbF.sub.5 as a fluorinating agent. Pacini et al., U.S. Pat. No. 3,287,424, teaches that SbF.sub.5 produces very vigorous reactions, sometimes resulting in carbon--carbon cleavage, also that dehydrohalogenation can take place in which double bonds are created. Similarly, Davis, U.S. Pat. No. 3,201,483, discloses that attempts to fluorinate normally easily fluorinated organic compounds with SbF.sub.5 usually result in highly exothermic reactions giving perfluorinated products.
Other related references are: Davis, U.S. Pat. No. 3,240,826 (1966), which utilizes SbF.sub.4 Cl, prepared by reaction of SbF.sub.5 with SbCl.sub.5, to replace Br with F in CF3CBr.sub.3, CF.sub.2 BrCHBr.sub.2 and CF.sub.3 CHBr.sub.2 at 35.degree.-120.degree. C.; and Davis, Canadian Patent 705,927 (1965), which employs SbF.sub.5 mixed with sufficient bromine to maintain SbF.sub.5 and salts thereof in liquid suspension for halogen exchange of fluorobromo compounds such as CF.sub.3 CHBr.sub.2 and CF.sub.2 BrCHBr.sub.2 at 20.degree.-70.degree. C.